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Are Pap Tests Accurate?

If you follow health news, you might have noticed some controversy over certain cancer-screening methods: Does the evidence support mammograms as a tool to reduce breast cancer deaths? Are PSA tests effective in saving lives from prostate cancer? These are questions that we are beginning to answer as more and more evidence comes in. But don’t let these questions dissuade you from all cancer screening.

In fact, although we’re reevaluating data for other cancer-screening methods, we have mountains of solid evidence that the Pap test is one of the best cancer-screening methods out there. Because it detects signature mutations that mark cells as headed toward becoming cancerous, Pap testing detects “pre” cancer while other cancer-screening techniques, like mammography, only detect cancer.

Cervical cancer used to be a top killer in developed nations — and it remains a major cause of death in countries without widespread health-care access — but in the last 50 years, cervical cancer deaths fell by 70 percent in the United States, transforming cervical cancer from the leading cause of cancer death among American women to a less common, nearly preventable cancer. Despite this, you might hear people complain that the Pap test isn’t accurate, citing the possibility of receiving “false positive” or “false negative” results.

A Pap test looks for abnormalities in cervical cells, and you can receive one of these four results:

True Positive: Cellular abnormalities are detected, and they are in fact present.

True Negative: Cellular abnormalities are not detected, and in fact the cells are normal.

False Positive: Cellular abnormalities are detected, but the cells are actually normal.

False Negative: Cellular abnormalities are not detected, but are actually present.

When we receive a true positive result, we can receive treatment for precancerous lesions that in fact might otherwise lead to cancer. Likewise, when we receive a true negative result, no further treatment is needed.

The false positive rate is low, between 1 and 10 percent of all positive results, but unfortunately, such a result can lead to more invasive treatment that might in reality be unnecessary. And a false negative result might allow precancerous cells to develop into cancer if they are not detected in a future round of Pap testing. Pap tests have a false negative rate of about 10 to 20 percent of all negative results, which might sound like a cause for alarm — but in practice, among those who receive regular Pap testing, cellular abnormalities that are missed once are, eventually, usually caught in time to be easily treated.

While it is true that Pap testing can lead to false positives or false negatives, the issue of its accuracy is a little more nuanced. In order to understand this issue, we need to understand the difference between “sensitivity” and “specificity.” What do these words mean?

sensitivity: a Pap test’s sensitivity refers to the likelihood of patients with cervical abnormalities receiving a true positive

specificity: a Pap test’s specificity refers to the likelihood of patients with no cervical abnormalities receiving a true negative

The sensitivity of a Pap test is about 70 to 80 percent. Does that mean that, if cancer is slowly developing on your cervix, there is up to a 30 percent chance Pap testing will miss it? The answer to that question is yes — if you never receive a Pap test again. They key to effective Pap testing is to undergo this screening regularly — current guidelines suggest every three years. And, because cervical cancer usually takes years to develop, if cancer is growing on your cervix, your chances of receiving a true positive result and being successfully treated are much higher than 70 to 80 percent if you’re receiving regular Pap tests. The “cumulative sensitivity” of Pap testing is very high, which is why cervical cancer deaths are so rare in the United States.

After the age of 30, people with cervixes have the option to combine HPV DNA testing with their Pap tests. The HPV DNA test has very high sensitivity and specificity — 94 percent! One study showed that when the HPV DNA test is used in combination with a Pap test, sensitivity was 100 percent and specificity was 92.5 percent. It is important to note that, while a Pap test and an HPV DNA test are separate laboratory procedures, the patient’s experience is the same: A health care provider collects cervical cells during a pelvic exam. You can’t have an HPV DNA test without the same physical exam required by a Pap test. HPV DNA and Pap co-testing are so much more accurate when performed simultaneously. Additionally, the sensitivity of liquid-based Paps is even better than that of traditional Pap tests, so ask your health care provider which method is being used.

Most false negatives — 90 to 95 percent of them — are due to inadequate sampling or improper slide preparation. You can take some steps to help your health care provider get an adequate sample:

likewise, try not to schedule your Pap test when you’ll be menstruating

for 48 hours prior to your Pap test, do not have sex, douche, or use tampons or vaginal creams, suppositories, medicines, sprays, or powders — any of these can wash away or hide cervical abnormalities, making them difficult to detect

Thanks to Pap testing, most cervical abnormalities never progress to full-fledged cancer. Among the 11,000 annual cases of cervical cancer that do develop in the United States, more than 50 percent of them occur in people who haven’t had a Pap test in at least five years — or ever. Because cervical cancer is caused by a common sexually transmitted pathogen — human papillomavirus (HPV) — Pap testing is important for sexually active, cervix-boasting folk.

Planned Parenthood health centers offer Pap testing as part of our commitment to preventive health care. Another way to reduce risk for cervical cancer is to be vaccinated with Gardasil, which protects against the two strains of HPV responsible for 70 percent of cervical cancers.

About Anna C.

Anna first volunteered for Planned Parenthood as a high school student in the 1990s. Since then, she has received a bachelor’s degree from UC Berkeley and a master's degree in epidemiology from the University of Arizona. As an ode to her fascination with microbes, she writes the monthly STD Awareness series, as well as other pieces focusing on health and medicine.